1. Field of the Invention
This invention relates to generators for generating electric power and more particularly to an exciter for such a generator having an automatic regulator and a manual regulator with a regulator selector which prevents switching from the automatic regulator to the manual regulator when the latter has failed.
2. Background Information
A synchronous electric power generator has a rotor with a field winding energized by an exciter through slip rings. Current in the rotor creates a rotating magnetic field which induces current in the stator to produce an output voltage at terminals of the stator. Originally, such machines had an exciter formed by a source and a rheostat which the operator manually adjusted to control the field current. Thus, such an arrangement was known as a manual control. Present systems have an arrangement which includes a transducer providing feedback of field current or field voltage which is used to automatically regulate the field current or voltage. As this system regulates the field current or voltage, as did the old manual control, it is still referred to as the xe2x80x9cmanual regulatorxe2x80x9d, even though it is automatic.
The modem systems also have a second regulator which regulates terminal voltage (on the stator) through feedback provided by a potential transformer (PT). This system is referred to as the xe2x80x9cautomatic regulatorxe2x80x9d and is the preferred control as it directly regulates terminal voltage, the primary controlled variable.
PT""s are known to fail. Also, they are protected by a fuse which can blow, thereby causing the automatic voltage regulator to fail. Present systems automatically switch to the xe2x80x9cmanual systemxe2x80x9d under these conditions. Also, under certain conditions the operator may want to switch from the automatic regulator to the manual system, such as for servicing or replacing the automatic regulator. However, it is also known that the transducer in the feedback loop of the manual system can fail. Typically, the operator has no indication of the transducer failure. Thus, a transfer could be made into the failed manual regulator resulting in total loss of exciter current. The resulting loss of generator output is unacceptable.
There is a need, therefore, for an improved synchronous generator, with a control system which prevents transfer to a failed manual voltage regulator.
This need and others is satisfied by the invention which is directed to an exciter system and an electric power generator incorporating such an exciter system having an automatic regulator and a manual regulator and a regulator selector which prevents switching from the automatic regulator to the manual regulator when the manual regulator has failed. More particularly, the invention is directed to an electric power generator comprising a rotor having a field winding, a prime mover rotating the rotor, a stator having a stator winding with output terminals, and an exciter providing field current to the field winding which induces current in the stator winding and produces an output voltage at the output terminals. The exciter comprises a first transducer providing a measure of field excitation and a potential transformer providing a measure of output terminal voltage. The exciter also includes a manual regulator for selectively providing the field current to the field winding as a function of the measure of field excitation. The exciter further includes an automatic regulator selectively providing the field current to the field winding regulated as a function of the measure of output terminal voltage. A regulator selector connects one of the manual regulator and the automatic regulator to provide the field excitation. This selector includes means responsive to failure of the manual regulator preventing switching from the automatic regulator to the manual regulator. For instance, the regulator selector includes logic responsive to loss of the field transducer excitation signal for preventing switching from the automatic regulator to the manual regulator. In the exemplary embodiment of the invention, the regulator selector includes AND logic for selecting the manual regulator. This AND logic has at one input a manual regulator selection signal and at a second input the negative of a signal indicative of a failure of the manual regulator. Thus, only in the absence of a signal indicative of the failure of the manual regulator can the manual regulator be selected.